1. Field of the Invention
This invention relates to an amplifier system for pre-amplifying a current or voltage waveform. More particularly, it relates to such an amplifier system for amplifying such waveforms of the type that are received from optical sensors, such as photodiodes or avalanche photodiodes. Most especially, it relates to such an amplifier system for pre-amplifying such a current waveform to form a voltage waveform, i.e. a transimpedance amplifier system.
2. Description of the Prior Art
In many optoelectronic applications, junction field effect transfer (JFET) into bipolar cascode amplifiers have become somewhat of a standard for low noise/high speed front ends, primarily because their immediate voltage gain desensitizes them to later stage noise voltage contributions. Other amplifier configurations are also used for these applications. Examples of prior art amplifier systems for such applications are disclosed in, for example, D.R. Smith et al., "High Performance Digital Optical Receivers with PIN Diodes," Proceedings of 1979 ISCAS, pp. 511-513 and M. J. N. Sibley et al., "A Monolithic Common-Collector Front-End Optical Preamplifier," Journal of Lightwave Technology, Vol. LT-3, No. 1, February 1985, pp. 13-15. To minimize noise over a wide bandwidth, input capacitance squared divided by the JFET transconductance (C.sub.T.sup.2 /g.sub.m) is kept small, as are photodiode and JFET leakage currents. Feedback resistor impedance (R.sub.L) is selected large so that its thermal noise current is small. Unfortunately, the bandwidth is a function of C.sub.T and R.sub.L and can become quite small for large R.sub.L and difficult to deal with in closed loop designs. Another disadvantage of the cascode front end is that recovery from amplifier saturation is hindered because some of the peak-to-peak voltage swing to remove charge from C.sub.T through R.sub.L is taken up biasing the second transistor. This is important in applications, such as bar code scanning, where the detector is exposed to signals with a wide dynamic range and ambient light.